I've never written a musical but I have acted in several so take that into consideration when you read this.
I'm not sure that you can write a musical that will stand alone as a play without the music (except for plays that use music for interludes).
In my experience, great musicals incorporate the music so that it is an integral part of the story. Several directors have told me that we sing when words are just not enough to convey the emotion of the moment. What would be left if you took the music out of Oklahoma! or South Pacific?
My gut feel is that if you wrote a play where the music was not an intrinsic part of the story you are telling, it wouldn't work as a musical.
As I said before there is the genre of "plays with music" - Foxfire with Hume Cronin and Fulghum's All I really Needed To Know I Learned In Kindergarten come to mind. They aren't true musicals so the removal of the music doesn't hurt the story all that much.
I guess a true musical is a play where the music can't be removed without a major rewrite. Oklahoma! and Green Grows The Grass is a case in point - although in that case the play existed before the musical.
Just my thoughts.
Doug